A photosensitive resin composition for image-forming has been largely used for applications such as various resist materials associated with electrical parts, printing boards and the like, since the resin composition enables a fine processing by applying a principal of a photographic method (photolithography) and can provide a cured product excellent in physical properties to form an image. There are a solvent development type and an alkali development type in the photosensitive resin composition for image-forming, and in recent years, the alkali development type which is capable of development with a diluted weak alkaline solution has become a mainstream from the viewpoint of an environmental protection and the alkali development type photosensitive resin composition is used also in, for example, production of a printed wiring substrate, production of a liquid crystal display board, printing plate making, and the like.
In the case that the photosensitive resin composition for image-forming is used in a process of a photographic method (photolithography) as, for example, a resin composition for a liquid development type solder resist, sequential steps of coating the resin composition on a substrate firstly, subsequently heat-drying to form a coating film, then applying a pattern forming film on the coating film, photo-irradiating and developing are employed. In such steps, when the coating film remains to have an adhesion after heat-drying, there have been problems such that a part of resist is adhered to the pattern forming film after peeling and an accurate pattern cannot be produced or the pattern forming film cannot be peeled. Therefore, a tack free property after forming the coating film is an important demanded characteristic of the liquid development type resist.
In addition, photosensitivity at photo-irradiation and developability after photo-irradiation are also important demanded characteristics. That is, in order to form a fine pattern with high reliability and good reproducibility, a portion cured by photo-irradiation is not allowed to be eroded with a developer in development, and on the contrary, an unexposed portion has to be readily removed in development.
Further, the cured portion is required to have characteristics such as heat resistance durable for a high temperature treatment to be subsequently carried out (soldering step and the like in the case of a solder resist) and long term reliability of water resistance, humidity resistance and the like.
As a solder resist satisfying the above described characteristics to some extent, a carboxyl group-containing epoxy(meth)acrylate introduced with a carboxyl group by reacting an acid anhydride with epoxy(meth)acrylate obtained by reacting an epoxy resin and (meth)acrylic acid has been known (for example, Patent Documents 1 and 2). This carboxyl group-containing epoxy(meth)acrylate satisfies conflicting characteristics such as tack free property, photosensitivity and developability with good balance, and in addition, has a good important characteristics such as heat resistance and water resistance required for a cured product, comparatively. However, characteristics of a higher level are required along with progress of technology, and for example, it is required not to deteriorate the accuracy of patterns which are miniaturized more and more and required to be a small dimensional change (low coefficient of linear expansion) against temperature change in order to improve crack resistance by meeting a coefficient of linear expansion to that of a substrate.
For the above described epoxy(meth)acrylate, it is considered that introducing a large number of double bonds in a resin structure using a multifunctional epoxy resin and (meth)acrylate to enhance a crosslinking density for the improvement of heat resistance and dimensional stability. However, since enhancing the crosslinking density results in becoming a cured coating film brittle, there is a problem that a balance between heat resistance and reduction in brittleness is hardly obtained. Regarding the reduction in brittleness, a technique of using a mixture of a bisphenol type carboxyl group-containing epoxy(meth)acrylate and a novolac type carboxyl group-containing epoxy(meth)acrylate is disclosed (Patent Document 3), however, flexibility is emphasized as described that the mixture can be applied to also a flexible wiring substrate, and there is a possibility to improve dimensional stability against temperature change and water resistance.    Patent Document 1: Japanese Unexamined Patent Publication (JP-A) No. 61-243869    Patent Document 2: Japanese Unexamined Patent Publication (JP-A) No. 63-258975    Patent Document 3: Japanese Unexamined Patent Publication (JP-A) No. 2000-109541